diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index 762509f6d4..4ed3ebf612 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -155,11 +155,11 @@ public static function processMachineName(&$element, FormStateInterface $form_st
     // ensure that the exists function is not called for existing values when
     // editing them.
     $initial_values = $form_state->get('machine_name.initial_values') ?: [];
-    $element_path = implode('][', $element['#parents']);
+    $element_key = implode('-', $element['#array_parents']);
     // Store the initial values in an array so we can differentiate between a
     // NULL default value and a new machine name element.
-    if (!array_key_exists($element_path, $initial_values)) {
-      $initial_values[$element_path] = $element['#default_value'];
+    if (!array_key_exists($element_key, $initial_values)) {
+      $initial_values[$element_key] = $element['#default_value'];
       $form_state->set('machine_name.initial_values', $initial_values);
     }
 
@@ -263,8 +263,8 @@ public static function validateMachineName(&$element, FormStateInterface $form_s
     // default value then it does not need to be validated as the machine name
     // element assumes the form is editing the existing value.
     $initial_values = $form_state->get('machine_name.initial_values') ?: [];
-    $element_path = implode('][', $element['#parents']);
-    if (!array_key_exists($element_path, $initial_values) || $initial_values[$element_path] !== $element['#value']) {
+    $element_key = implode('-', $element['#array_parents']);
+    if (!array_key_exists($element_key, $initial_values) || $initial_values[$element_key] !== $element['#value']) {
       $function = $element['#machine_name']['exists'];
       if (call_user_func($function, $element['#value'], $element, $form_state)) {
         $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
diff --git a/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php b/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php
deleted file mode 100644
index e09ca6cf4c..0000000000
--- a/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php
+++ /dev/null
@@ -1,57 +0,0 @@
-<?php
-
-namespace Drupal\Tests\system\FunctionalJavascript;
-
-use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
-
-/**
- * Tests validation of machine name elements.
- *
- * @group Form
- */
-class MachineNameValidationTest extends JavascriptTestBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected static $modules = ['form_test'];
-
-  /**
-   * Tests that a machine name is still required after an AJAX submit.
-   *
-   * This guards against the regression in https://www.drupal.org/node/2557299.
-   */
-  public function testMachineNameRequiredFormAjaxSubmit() {
-    $assert = $this->assertSession();
-    $this->drupalGet('/form-test/form-test-machine-name-validation');
-
-    // Test errors after with no AJAX.
-    $assert->buttonExists('Save')->press();
-    $assert->pageTextContains('Machine-readable name field is required.');
-    // Ensure only the first machine name field has an error.
-    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
-    $this->assertFalse($assert->fieldExists('id2')->hasClass('error'));
-
-    // Test a successful submit after using AJAX.
-    $assert->fieldExists('Name')->setValue('test 1');
-    $assert->fieldExists('id')->setValue('test_1');
-    $assert->selectExists('snack')->selectOption('apple');
-    $assert->assertWaitOnAjaxRequest();
-    $assert->buttonExists('Save')->press();
-    $assert->pageTextContains('The form_test_machine_name_validation_form form has been submitted successfully.');
-
-    // Test errors after using AJAX.
-    $assert->fieldExists('Name')->setValue('duplicate');
-    $this->assertJsCondition('document.forms[0].id.value === "duplicate"');
-    $assert->fieldExists('id2')->setValue('duplicate2');
-    $assert->selectExists('snack')->selectOption('potato');
-    $assert->assertWaitOnAjaxRequest();
-    $assert->buttonExists('Save')->press();
-    $assert->pageTextContains('The machine-readable name is already in use. It must be unique.');
-    // Ensure both machine name fields both have errors.
-    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
-    $this->assertTrue($assert->fieldExists('id2')->hasClass('error'));
-
-  }
-
-}
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Core/MachineNameTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Core/MachineNameTest.php
index bf2643da6a..acbaadb698 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Core/MachineNameTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Core/MachineNameTest.php
@@ -59,7 +59,6 @@ public function testMachineName() {
 
     // Get page and session.
     $page = $this->getSession()->getPage();
-    $assert_session = $this->assertSession();
 
     // Get elements from the page.
     $title_1 = $page->findField('machine_name_1_label');
@@ -111,6 +110,36 @@ public function testMachineName() {
 
     // Validate if the element contains the correct value.
     $this->assertEquals($test_values[1]['expected'], $machine_name_1_field->getValue(), 'The ID field value must be equal to the php generated machine name');
+
+    $assert = $this->assertSession();
+    $this->drupalGet('/form-test/form-test-machine-name-validation');
+
+    // Test errors after with no AJAX.
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('Machine-readable name field is required.');
+    // Ensure only the first machine name field has an error.
+    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
+    $this->assertFalse($assert->fieldExists('id2')->hasClass('error'));
+
+    // Test a successful submit after using AJAX.
+    $assert->fieldExists('Name')->setValue('test 1');
+    $assert->fieldExists('id')->setValue('test_1');
+    $assert->selectExists('snack')->selectOption('apple');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The form_test_machine_name_validation_form form has been submitted successfully.');
+
+    // Test errors after using AJAX.
+    $assert->fieldExists('Name')->setValue('duplicate');
+    $this->assertJsCondition('document.forms[0].id.value === "duplicate"');
+    $assert->fieldExists('id2')->setValue('duplicate2');
+    $assert->selectExists('snack')->selectOption('potato');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The machine-readable name is already in use. It must be unique.');
+    // Ensure both machine name fields both have errors.
+    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
+    $this->assertTrue($assert->fieldExists('id2')->hasClass('error'));
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php b/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php
index 3cbbab0ac2..794f2b129d 100644
--- a/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php
@@ -62,7 +62,7 @@ public function testProcessMachineName() {
       ],
       // The process function requires these to be set. During regular form
       // building they are always set.
-      '#parents' => ['test_machine_name'],
+      '#array_parents' => ['test_machine_name'],
       '#default_value' => NULL,
     ];